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Great AP reaction

Great AP reaction

So I'm driving on the freeway in rush hour yesterday afternoon. Traffic is stop and go, but just starting to clear out so I'm going about 40 mph. My exit is coming up so I'm in the right lane on AP with a follow distance of 1. Suddenly my car brakes, fairly abruptly. Over the next fraction of a second my thoughts were:

I don't see any brake lights on the SUV in front of me.
I didn't just pass under an overpass, and it's overcast with light rain so I shouldn't get any phantom braking.
Maybe I should press the accelerator to keep up with traffic & not get hit from behind.

It was at that instant that the SUV in front slammed on it's brakes. I then see the car in front of them, which had been obscured from my view, braking hard and pulling onto the shoulder to avoid plowing the car in front of them (3 cars in front of me).

So my car reacted to the car either 2 or 3 cars ahead, which I couldn't even see. Amazing.

EVRider | 2020年1月17日

Yes, and even HW1 could do that. The radar goes under the car in front of you to detect a car in front of that one. I remember someone posting their dashcam video of a similar incident 2-3 years ago.

derotam | 2020年1月17日

It's a WITCH! :)

M3phan | 2020年1月17日

@ derotam, ...I got better...

@ Effopec, excellent report!

jimglas | 2020年1月17日

I have also experienced this
Pretty amazing

EVRider | 2020年1月17日

Here's one of the many YouTube videos that shows this: https://youtu.be/APnN2mClkmk

The accident happens at the beginning of the video, nothing interesting after that.

thedrisin | 2020年1月17日

@EVRider. How can radar go under the cars in front of you since Model 3 cannot detect obstacles that are located lower than the bumper.

M3phan | 2020年1月17日

@ thedrisin, from Tesla, https://www.tesla.com/autopilot, the front facing radar has a distance of 160 meters and “...is able to see through heavy rain, fog, dust and even the car ahead.” The radar isn’t used for parking. The sensors are.

thedrisin | 2020年1月17日

@M3phan. Why is it then that under summon it says "Warning: Model 3 cannot detect obstacles that are located lower than the bumper, "

M3phan | 2020年1月17日

Because summon and smart summon does not use radar

peldor427 | 2020年1月17日

I think the issue of seeing things lower than the bumper is the proximity when parking as well. I usually have a correct distance warning shown for low curbs that goes away when I get within about 30-40", maybe because of the sharp downward angle required to see them. The warnings then focus on the next object that isn't as low.

So maybe it can see underneath a car when it's a less sharp angle because the 2nd car is much farther away than a curb would be.

jamilworm | 2020年1月17日

I feel ike I remember reading that the ability to see 2 cars ahead is because the radar bounces off the ground under the car in front to see the next car. So even if the car in front of you has a lower clearance than the car 2 ahead and there is no direct line of sight, a Tesla can still detect it.

Tronguy | 2020年1月17日

@thedrisin: For what it's worth, I'm really, no kidding, a radar guy. I used to fix them for a living; we're talking AWACS here. And I've had the college courses and, of course, the degree.
The Tesla sends out radar pulses, They hit things and the reflections come back. Note: The radar beam going out is not like a little bitty LASER, it's probably on the order of 10 degrees wide and conical in shape.
So, when a pulse goes out, it's going to hit stuff and reflect back. If there are five things at different distances out there, five reflected return pulses will return at times proportional to the distance.
Now, @M3Phan says that the radar has a range of 160 meters or so. That can be due to one or both of the following reasons:
1. The pulse repetition rate. The longer between pulses, the longer one is listening for reflections and the range is longer. Let's see, radar range for 160 meters is 320meters/3e8 meters per second = 1.07 us; that sounds a bit fast. It's probably more like 10 us, and that may be faster in terms of processing than the computer can go.
2. Sensitivity. An airborne radar is trying to pick up targets 300 miles away so its antenna is huge, the receiver sensitive as all get out, and the power high enough to fry seagulls at 100 meters. None of that is going to be true for a car traipsing down the freeway. Power levels are likely in the low milliwatts, the better not to fry people, the antenna is small, and really sensitive receivers are expensive and unwarranted. Not to mention that one doesn't want to interfere with the radar receivers on other cars. That might be cured with a dose of a unique pattern of frequency hopping on the radar's part, so if several radar reflections don't get picked up at the same spot as the radar hops around, it'll not be there.
So, I'm going for #2. Figure that the pulse width is 15 or 20 ns wide to give good resolution, and there we are.
No problem with detecting cars two ahead: Reduced magnitude of the reflections, of course, but most decent radar receivers can pick stuff up in the microwatts. And we have, at speed, a low-pass filter on the homodyne radar receiver in the thing that kills clutter, making the doppler-shifted road kind of disappear for detection purposes.
Ha. And you guys thought that cell phones were complicated...

kevin_rf | 2020年1月18日

I think you hit on a key, it is probably using the Doppler shift to filter the different cars, road, and on coming cars. Mostly likely as you said, a 1d sensor that needs the forward facing camera to help assign and filter the different detections. The Doppler should tell it if the cars in front are slowwing or speeding up.

I've always suspected the filtering plays a significant role in the phantom braking. As in it assigns the wrong return to the wrong object in the camera field of view.

EVRider | 2020年1月18日

Here’s an old Tesla blog entry from Sept 2016 explaining how radar is used: https://www.tesla.com/blog/upgrading-autopilot-seeing-world-radar

The last paragraph talks about bouncing radar under the car in front of you to “see” a car (or UFO) in front of that one.

Sarah R | 2020年1月18日

@Effopec But you forgot to save the video!